The glycophorin A (GPA)-based human in vivo somatic cell mutation assay, because of its demonstrated long-term biological memory of past genotoxic exposures together with its relatively low cost and high sample thruput, is unique among presently available human biomarker assays for practical use as a retrospective biodosimeter in epidemiological investigations of large human populations. Here we propose to further validate the response of the assay as a biodosimeter of radiation exposure in a longitudinal study of patients receiving 131I therapy for thyroid disease. These patients will receive clinically well-characterized doses ranging from 2 to 200 cGy of whole body bone marrow exposure to ionizing radiation resulting from the radioactive decay of administered 131I. Peripheral blood samples from approximately 100 patients will be drawn prior to, during, and following therapy to follow the induction, accumulation, and persistence of radiation-induced somatic mutation at the GPA locus in bone marrow stem cells. These mutations in nucleated bone marrow progenitor cells give rise to erythrocytes in the peripheral circulation expressing a GPA allele-loss variant phenotype. These variants are directly enumerated in the assay using immunolabeling with GPA allele-specific monoclonal antibodies and flow cytometry. Based on GPA assay results obtained in high radiation dose populations, this study is designed to investigate the radiation-dose- response of the assay over a range of doses that surround the extrapolated doubling dose over background response of the assay of approximately 30 cGy. The longitudinal design of the study, applied to patients receiving relatively low doses of 131I, will permit a test of the practical ultimate sensitivity of the assay by comparing GPA variant cell frequencies in post- therapy samples to those observed in pre-therapy samples within individual patients. This design, where a repeated measures analysis can be applied to individual responses, avoids the limitations inherent in studies using exposed versus matched control populations where inter-individual variability in response and in background frequencies limit the power of the assay to detect small increases in variant cell frequencies induced by low-dose exposures. These data will provide critical information to assess the power of the assay to demonstrate, or to estimate the upper limits of radiation exposures in population surveys of occupationally-exposed workers. In addition, since a number of patients receiving very similar 131I doses will enrolled in the study, the design will also permit an investigation of the range of individual responses in the assay to similar exposures. These data will also contribute significantly to population risk assessment models that attempt to include this heterogeneity as well as narrowing the range of uncertainty surrounding individual estimates of received radiation dose inferred from individual responses in the assay.